The role of chromosomal proteins in maintaining the structure and regulating the function of chromatin and chromosomes is being studied. Specific antibodies were used to construct immunoaffinity columns for fractionating chromatin. Studies on the exchange of proteins during immunofractionation of chromatin revealed that at low ionic strength there was negligible exchange of proteins between nucleosomes. Nucleosomes enriched in HMG-17 or H1o have been isolated by immunoaffinity chromatography and the DNA present in these nucleosomes examined with a variety of genetic probes. The results indicate that chromatin fragments containing DNA sequences with an open reading frame are enriched in HMG-17 while nucleosomes containing DNA sequences coding for inducible proteins are depleted of H1o. We cloned and sequenced cDNA coding for human chromosomal protein HMG-17 and used this cDNA to probe the genomic organization of the gene. In the hunan genome, there are over 50 gene equivalents for this cDNA suggesting that the protein is encoded by a multigene family. Southern analysis of the DNA from several transformed human cell lines failed to detect any restriction fragment polymorphism in this gene. The cDNA has some unusual characteristics: only 25% of the transcript is translated, the 5' untranslated region is extremely rich in GC residues, while the 3' untranslated region is very rich in AT residues.